1. Field of the Invention
The present invention is directed toward a method for controlling a vehicle heating, ventilation, and air conditioning (HVAC) system. In particular, the present invention relates to automatically sensing fogging conditions, and instituting appropriate measures to prevent fogging and to remove window fog should fog be present.
2. Description of Related Art
In modern vehicles it is common to have an HVAC system for a user compartment (hereinafter “cabin”). The HVAC system provides warm and cool air to the cabin of the vehicle. The HVAC system allows users to select a set temperature for the cabin. Once the set temperature is selected, the HVAC system will provide conditioned air to adjust the climate of the cabin to the set temperature. Further, the HVAC system is used to eliminate fog or condensation that appears on the vehicle windows and reduces or eliminates visibility into or out of the cabin.
In conventional automobile control systems, application of remedial measures to combat fogging of the vehicle windows is manually instituted by the vehicle passengers. Naturally, such remedial measures, which primarily consist of activating the HVAC system into a defrost mode of operation, are employed after a fogging condition has developed, and typically require some time to alleviate the fogging condition. As will be appreciated, such systems are not suitable for automatic control, and do not anticipate impending fogging conditions.
In vehicles incorporating an automatic HVAC control system, it has been proposed to sense internal and external conditions to determine whether fogging is likely. Such known systems include a glass temperature sensor and a humidity sensor. The glass temperature sensor is affixed to an internal surface of the windshield glass, and the humidity sensor is disposed adjacent to the windshield glass. The system uses the sensed glass temperature and the sensed cabin humidity to determine whether a possible fogging condition exists, and actuates the HVAC system in response thereto. Unfortunately, such automated systems require additional sensors, which are expensive to purchase and require significant amounts of labor to install and electrically connect to a controller.
An alternative automatic HVAC control system senses cabin temperature, cabin humidity, ambient temperature, and vehicle speed. Such a system calculates a cabin dewpoint based upon the cabin temperature and cabin relative humidity. Further, the system estimates a glass temperature based upon the ambient temperature and the vehicle speed. Then, a fog margin, which is the difference between the cabin dewpoint and the estimated glass temperature, is determined. The lower the value of the fog margin, the more likely that fogging of the windshield will occur. The fog margin is used to accordingly control the HVAC system.
Unfortunately, this alternative automatic control does not take into consideration the presence of precipitation, such as rain or snow, outside of the vehicle. Nor does the alternative automatic control take into consideration when the cabin temperature is less than a predetermined value, such as for example, when the user initially enters the cabin on a cold day. During this condition, which is known as cabin soaking, the cabin temperature is less than the predetermined value and the windshield is relatively more susceptible to fogging. Accordingly, precipitation or cabin soaking can influence the appearance of fog on the windows.
Therefore, there exists a need in the art for a method to better control the HVAC system so as to prevent fogging during precipitation or soaking conditions.